1. Field of the Invention
The present invention relates to a uniform flash-emission controller which controls a flash, or a strobe, so as to give off light with a uniform intensity for a given period of time via intermittent driving of an IGBT (Insulated Gate Bipolar transistor), wherein the IGBT is rapidly switched ON and OFF repetitively.
2. Description of the Related Art
A flash/strobe (e.g., a speedlite used for cameras) which is controlled so that the flashtube (e.g., a xenon flashtube) thereof gives off light with a uniform intensity for a given period of time is known in the art. Note that the use of the term xe2x80x98uniform intensityxe2x80x99 or xe2x80x98uniform flash emissionxe2x80x99 refers to one kind of flash emission control used in high-speed synchronized photography, and can be also referred to as xe2x80x98flat emissionxe2x80x99. Such a control is referred herein as a uniform flash-emission control. In the uniform flash-emission control, it is generally the case that the intensity of the light emission of the flashtube is controlled by an intermittent drive of an IGBT (Insulated Gate Bipolar transistor) wherein the IGBT is switched ON and OFF repetitively. Namely, the light emitted by the flashtube is received by a light-receiving element to be converted into a voltage signal, and the IGBT is switched ON and OFF repetitively in accordance with the output of a comparator which compares the voltage signal with a predetermined voltage. In this type of uniform flash-emission control using the IGBT, the IGBT needs to be switched ON and OFF at high speed. The flash can give light emission on the subject with less fluctuation in intensity as the control frequency for switching the IGBT ON and OFF becomes higher.
However, if the control frequency for the IGBT is high, the power loss due to the IGBT increases, and also the control frequency may exceed the maximum operable frequency of the IGBT. Accordingly, the IGBT gets damaged if the control frequency for the IGBT exceeds the maximum operable frequency thereof.
The object of the present invention is to provide a uniform flash-emission controller which controls a flash to give off light with a uniform intensity for a given period of time via an intermittent drive of an IGBT, wherein the IGBT is prevented from being damaged during the uniform flash-emission control.
To achieve the object mentioned above, according to an aspect of the present invention, a uniform flash-emission controller is provided, which controls an intensity of a light emission of a flashtube, the uniform flash-emission controller including an IGBT which causes the flashtube to emit a rapid series of short flash pulses; an IGBT controller which switches the IGBT ON and OFF so as to maintain the intensity at a substantially constant level; and a latch for holding an ON state and an OFF state of the IGBT until a predetermined period of time elapses from a time the IGBT controller switches the IGBT ON and OFF, respectively.
Preferably, the IGBT controller includes a detecting device for detecting the intensity of the light emission of the flashtube. The IGBT controller switches the IGBT OFF at a time the intensity detected by the detecting device exceeds a first predetermined intensity, and subsequently switches the IGBT ON at a time the intensity detected by the detecting device drops below a second predetermined intensity.
In an embodiment, the first predetermined intensity is identical to the second predetermined intensity.
According to another aspect of the present invention, a uniform flash-emission controller is provided, including an IGBT which causes the flashtube to emit a rapid series of short flash pulses; an IGBT controller which switches the IGBT ON and OFF so as to maintain intensity of a light emission of the flashtube at a substantially constant level; a detecting device for detecting the intensity of the light emission of the flashtube; a comparator which compares the intensity of the light emission detected by the detecting device with a predetermined intensity, and outputs a level signal responsive to the intensity of the light emission detected by the detecting device; a latch which latches the level signal, and outputs the level signal as one of an ON signal and an OFF signal to the IGBT to switch the IGBT ON and OFF, respectively; a switching device provided between the comparator and the latch; and a switch controller for holding the switching device in an OFF state so that one of the ON signal and the OFF signal, which is output from the latch, cannot change until a predetermined period of time elapses from the moment the level signal changes.
In an embodiment, the comparator outputs a high-level signal and a low-level signal in the case where the intensity of the light emission detected by the detecting device is greater and less than the predetermined intensity, respectively. The IGBT controller switches the IGBT ON to thereby cause the flashtube to emit light when the latch latches the high-level signal to output the ON signal to the IGBT. The IGBT controller switches the IGBT OFF to thereby cause the flashtube to stop emitting light when the latch latches the low-level signal to output the OFF signal to the IGBT.
In an embodiment, the predetermined period of time corresponds to a maximum operable frequency of the IGBT.
Preferably, the switching device includes a first buffer which inputs the level signal output from the comparator, the first buffer including an input terminal which inputs a signal output from the switching controller.
Preferably, the latch includes a second buffer, and a resistor which is connected between input and output terminals of the second buffer so that an output of the second buffer is fed back to the input terminal of the second buffer via the resistor.
Preferably, the switch controller includes an RC circuit which is connected to an output port of the latch, the predetermined period of time being determined by a time constant of the RC circuit.
Preferably, the uniform flash-emission controller further includes a flash controller which determines the intensity of the light emission of the flashtube and a duration of the light emission of the flashtube to control a commencement of the light emission of the flashtube and a termination of the light emission of the flashtube.
Preferably, the flash controller outputs a light-emission stop signal for terminating the light emission of the flashtube to the latch via the switching device upon a lapse of the duration of the light emission of the flashtube; wherein the light-emission stop signal is output to the IGBT via the latch without delay when the switching device is in an ON state. When the switching device is in the OFF state, the light-emission stop signal is output to the IGBT upon a change of a state of the switching device from the OFF state to the ON state after the predetermined period of time elapses.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-260632 (filed on Aug. 30, 2000), which is expressly incorporated herein by reference in its entirety.